The present invention is an electronic circuit for demodulating servo information read from a magnetic storage medium. More specifically, the present invention is a demodulation circuit used in a digital video cassette recording system for demodulating embedded frequency modulated servo schemes from a magnetic storage medium, and thereby determining the position of a read head with respect to a specific track of the magnetic storage medium.
A video cassette can include two types of information on the magnetic storage medium, such as a magnetic tape. First, the storage medium can include data information which can be written to or read from the storage medium by electrical circuitry within a video cassette recorder (VCR) and which can be manipulated by the electrical circuitry within the VCR to recover the data from the storage medium. Second, the video cassette can include servo information, which is information which can be recovered from the magnetic storage medium by electrical circuitry within the VCR to monitor and position a read head of the VCR. The servo information is utilized by the VCR to position and center the read head above the desired track of data information on the magnetic storage medium such that the read head will properly read the data from the track. If the read head is not properly centered over the desired track of the magnetic storage medium, circuitry within the VCR will utilize the servo information to alter the position of the read head such that it is properly centered above the desired track.
Presently there are two positions on a magnetic storage medium in which servo information is written. First, servo information can be written at the beginning of each track of the magnetic storage medium. In this type of system, the servo information is also called servo bursts, meaning that there is a burst of information positioned at the beginning of each track containing no data information, just servo information. These servo burst areas are a source of interruption to data information storage.
Second, servo information can be written on alternating tracks such that a first track would include data as well as embedded servo information and a second track would include data information only. While reading down the data-only track the read-head can also read the embedded servo information on the two adjacent tracks as a fringing field. This information is then used to center the read-head on the track. The technique of storing and subsequently recovering servo information from alternating tracks has become the standard in digital video cassette recording systems.
One type of prior art VCR system uses high-Q or very tight bandpass, continuous-time filters to extract servo information from the recovered data. Given the typical component tolerances in any standard silicon IC fabrication process, a high-Q bandpass, continuous-time filter is usually difficult to control. In order to achieve accurate control over parameters such as the filter cut-off frequency, elaborate control circuitry needs to be included on the same chip. The resulting system complexity is often high leading to an increase in the overall cost as well as power consumption. An alternate is to emulate the filtering function using Digital Signal Processing (DSP) technique. However, this alternative also results in an unacceptably high-cost high-complexity implementation for most applications. Yet another option is to lock onto the frequency-modulated servo information using a Phase-Locked Loop (PLL). For such a system to function, the PLL circuitry needs to have a very narrow lock-range leading to an unacceptably long transient response time for the overall system.
Therefore, there is a need for an automatic tracking frequency servo demodulation technique for digital VCR applications which is not hardware intensive and which provides a relatively fast response.